Viral infection of cells has been shown in many instances to be accompanied by karyotypic changes (Hampar and Ellison, 1963; Nichols et al., 1962; Shein and Enders, 1962; Koprowski etal., 1962; Black and Rowe, 1963) , so that the chromosomal alterations observed at the initiation of continuous cell lines may be secondary to the real cause of the phenomenon.